The Mixed Lineage Leukemia (MLL) gene at chromosome band 11q23 is commonly involved in reciprocal translocations detected in acute leukemias. A number of experiments show that the resulting MLL fusion genes directly contribute to leukemogenesis. Among the many known MLL fusion partners, AF4 is relatively common, particularly in acute lymphoblastic leukemia in infants. The AF4 protein interacts with the product of another gene, AF9, which is also fused to MLL in acute leukemias.
Eighty percent of cases of acute lymphoblastic leukemia (ALL) that occur in infants (children less than one year of age) have translocations involving the MLL gene at chromosome 11q23. MLL is an ortholog of the Drosophila gene Trithorax. Trithorax and MLL gene products play essential roles in embryogenesis, in part by binding the promoters of HOX genes and maintaining the expression of these genes (Yu, B. D., et al., Nature 1995 378: 505–508; Yu, B. D., et al., Proc Natl Acad Sci USA. 1998 95: 10632–10636; Hess, J. L., et al., Blood 1997 90: 1799–1806; and Milne, T. A., et al., Mol Cell 2002 10: 1107–1117). While MLL is rearranged in the majority of infants with leukemia, translocations involving MLL are encountered in all age groups and are particularly common in secondary leukemias that arise following exposure to epipodophyllotoxin-containing chemotherapy regimens (Rowley, J. D. Annu Rev Genet 1998 32: 495–519).
Remarkably, in acute leukemias characterized by MLL gene rearrangements, a portion of MLL is rejoined to any one of nearly 40 different loci. The rearranged gene is expressed and chimeric MLL proteins have been detected in cells (Li, Q., et al., Blood 1998 92: 3841–3847). MLL “partner” genes are surprisingly heterogeneous, but some of them are known to encode transcription factors or proteins with transcriptional activity. Fusion of transcription factors to MLL has been proposed as one mechanism of triggering leukemia by inappropriately activating crucial developmental genes (Ayton, P. M., and Cleary, M. L. Oncogene 2001 20: 5695–707; So, C. W., and Cleary, M. L. Blood 2003 101: 633–639; and Zeisig, B. B., et al, Leukemia 2003 17: 359–65.).
Regardless of the mechanism, there is compelling data to suggest that the MLL fusion proteins are important in the pathogenesis of the disease. First, different MLL fusion proteins are non-randomly associated with specific subtypes of leukemia (Rowley, cited above; Mitelman, F., Mutat Res 2000 462: 247253). Second, retroviral expression of MLL fusion genes causes leukemic transformation of hematopoietic progenitor cells (Lavau, C., et al., EMBO J 1997 16: 4226–4237; DiMartino, J. F., et al., Blood 2000 96: 3887–3893; and Lavau, C., et al., Proc Natl Acad Sci USA 2000 97: 10984–10989). Third, a chimeric knock-in mouse expressing an MLL-AF9 fusion gene develops acute myeloid leukemia (AML) with the characteristics of MLL-AF9 leukemia that occurs naturally in humans (Corral, J., et al., Cell 1996 85: 853–861).
Despite the large number of MLL fusion genes, in more than 50% of infants with ALL, the leukemic blast cells contain the reciprocal translocation t(4;11)(q21;q23) and are associated with a distinctive CD1031 CD19+ ALL phenotype. As a consequence of this t(4;11) translocation, the 5′ portion of the MLL gene at 11q23 is fused to the 3′ portion of a gene at the 4q21 locus designated AF4. Although less common, the next most frequently encountered translocations in infant ALL are t(11;19)(q23;p13) and t(9;11)(p22;q23) (Felix, C. A., et al., Hematology (Am Soc Hematol Educ Program) 2000 285–302; and Pui, C. H., et al., Leukemia 2003 17: 700–6). In these cases, the ENL and AF9 genes respectively are joined to MLL. ENL and AF9 are structurally related proteins and the 3′ portion of the genes that fuse to MLL encode nearly identical amino acid sequences (Slany, R. K., et al., Mol Cell Biol 1998 18: 122–129).
Apart from frequent involvement in acute leukemia when expressed as MLL fusion proteins, the biological functions of AF4, ENL and AF9 are not well understood. Like MLL, mouse gene deletion studies have demonstrated important developmental roles for AF4, AF9 and ENL (Isnard, P., et al., Blood 2000 96: 705–710; Collins, E. C et al., Mol Cell Biol 2002 22: 7313–7324; and Doty, R. T., et al., Blood Cells Mol Dis 2002 28: 407–417). AF4 and AF9 form a stable protein complex in the nucleus and the mutual interaction domains of the two proteins are present within MLL fusion proteins (Erfurth, F., et al., Leukemia 2004 18: 92–102). This observation raises the possibility that AF4 and AF9 function in tandem both in their native states and when expressed as a chimeric MLL protein.
Despite considerable advances in the treatment of acute lymphoblastic leukemia in children, ALL in infants remains a particularly challenging disease. Recent clinical trials have produced outcomes with <50% event free survival after 5 years. The prognosis may be even more limited for the large number of infants with t(4;11) leukemia (Felix, C., and Lange, B. J., Oncologist 1999 4: 225–240; Reaman, G. H., et al., J Clin Oncol 1999 17: 445–455; Biondi, A., et al., Blood 2000 96: 24–33; Chessells, J. M., et al., Leukemia 2002 16: 776784). Current treatments for infant ALL include high dose chemotherapy with or without stem cell transplant and are associated with high morbidity in addition to the possibility of disease relapse. An optimal treatment regimen for these high-risk patients is yet to be identified (Pui, C. H et al., Lancet 2002 359: 1909–1915).
There is a great interest in expanding the range of therapies for infant leukemias and an understanding of the biology of the disease has provided some leads. To date, however, no effective disease-specific agent for the treatment of babies with ALL has been described. Thus, there remains a need in the art for compounds and pharmaceutical compositions and methods useful for treatment, prevention and diagnosis of a variety of leukemias, including infant leukemias and secondary leukemias following chemotherapy.